C 2.2 SL Neural Signaling (PDF)

C 2.2 Neural Signaling “How are electrical signals generated and moved within neurons?” “How can neurons interact with other cells?” earning Objectives Students should understand that cytoplasm and a nucleus Neurons as cells within...

C 2.2 Neural Signaling “How are electrical signals generated and moved within neurons?” “How can neurons interact with other cells?” earning Objectives Students should understand that cytoplasm and a nucleus Neurons as cells within form the cell body of a neuron, with elongated nerve fibres C2.2. the nervous system that of varying length projecting from it. An axon is a long 1 carry electrical impulses single fibre. Dendrites are multiple shorter fibres. Electrical impulses are conducted along these fibres. Generation of the resting Students should understand how energy from ATP drives potential by pumping to the pumping of sodium and potassium ions in opposite C2.2. establish and maintain directions across the plasma membrane of neurons. They 2 concentration gradients should understand the concept of a membrane polarization of sodium and potassium and a membrane potential and also reasons that the ions resting potential is negative. Nerve impulses as action Students should appreciate that a nerve impulse is C2.2. potentials that are electrical because it involves movement of positively 3 propagated along nerve charged ions. fibres Compare the speed of transmission in giant axons of squid C2.2. Variation in the speed of and smaller non-myelinated nerve fibres. Also compare the 4 nerve impulses speed in myelinated and non-myelinated fibres. Synapses as junctions Limit to chemical synapses, not electrical, and these can C2.2. between neurons and simply be referred to as synapses. Students should 5 between neurons and understand that a signal can only pass in one direction effector cells across a typical synapse. Release of Include uptake of calcium in response to depolarization of C2.2. neurotransmitters from a a presynaptic membrane and its action as a signalling 6 presynaptic membrane chemical inside a neuron. Neurons as cells within the nervous system that carry electrical impulses The nervous system consts of nerve cells called neurons. The neurons help with internal communication by transmitting nerve impulses. Neurons as cells within the nervous system https://alienhippy.files.wordpress.com/2012/04/cognifit- that carry electrical impulses Neurons transmit information along nerve fibers in the form of electrical impulses. The electrical impulse is not like an electrical current that flows along wires. An impulse is a momentary reversal in electrical potential difference in the membrane – a change in the position of charged ions between urons as cells within the nervous system ou annotate the structures of the neu https://en.wikipedia.org/wiki/Soma_%28biology%29 The neuron is the basic functional unit of the nervous system. Neurons have a cell body with cytoplasm and a nucleus, as well as a long narrow outgrowth called nerve fibers along Generation of the resting potential A nerve impulse is a result of a change in concentration of sodium (Na+) and potassium (K+) ions along the cell membrane. Depending on the membrane potential (voltage), we can distinguish between a resting potential and an action potential. Resting potential: this is the potential difference across a nerve cell membrane when it is not stimulated. It is at approximately -70mV. Action potential: This is the reversal (depolarisation) and restoration (repolarisation) of the electrical potential across a plasma membrane as a nerve impulse passes along a neuron. urons as cells within the nervous system Generation of the resting potential All cells have a measurable membrane potential (voltage), which is established by the imbalance of ions inside and outside of a cell. The membrane potential in neurons is – 70mV. 3 factors contribute to this value: 1 Sodium-potassium pumps 2 Leakage of ions back across the membrane by simple diffusion 3 Negatively charged proteins inside the Generation of the resting potential Sodium-potassium pumps in the membrane transfer Na+ out of the neuron and K+ into the neuron. This is active transport and requires ATP. The number of ions pumped is inequal – three Na+ go out, two K+ ions go in. This causes an imbalance of ions and concentration gradients for both.  Generation of the resting potential Leakage of potassium and sodium ions also https://classconnection.s3.amazonaws.com/981/flashcards/598981/jpg/ contributes to the imbalance of ions across the membrane. The axon membrane has a higher permeability of K+, which leaks out of the cell through its membrane channels faster than Na+ leaks in through its Na+ protein channels. Generation of the resting potential Due to this unequal distribution of ions, and the abundance of negatively charged proteins inside the axon fibre, the membrane potential is found to be at ca. -70mV. http://www.highlands.edu/academics/divisions/scipe/biology/faculty/harnden/2121/images/ Nerve impulses as action potentials ction potential is composed of two subsequent stages: Depolarization: The cell membrane's charge becomes positive. This is caused by positive sodium ions going Repolarization: Theinto cellthe cell. membrane's charge returns to negative. This is caused by positive potassium ions moving out of the cell. http://faculty.southwest.tn.edu/rburkett/A&P1_m30.jpg Nerve impulses as action potentials Depolarization Repolarization A stimulus causes sodium ions Towards the end of the action (Na+) to flow into the potential the flow of sodium ions cytoplasm of the axon, stopps and potassium channels open reversing the polarity of the up. This causes the flow of axon. This makes the potassium ions (K+) out of the axon, membrane potential more bringen the membrane potential positive (from -70mV to back down to -70mV. +40mV). Nerve impulses as action potentials The action potential then progresses along the whole length of the axon fiber.  https://www.researchgate.net/figure/The-squid-giant-axon-The-giant-axon-is-a-very-large-up-to-1-mm-in-diameter-and- Variation in the speed of nerve impulses Signal transmission in humans is at about 1m per second. This speed is largely due to the small diameter (1µm). Some animals have nerve fibres with larger diameters. A bigger diameter reduces resistance and therefore increases conduction speed. Giant axons in squids have diameters of up to 500 µm. How does this help the squid as an adaptation to life? Variation in the speed of nerve impulses Read the article and answer the questions on your sheet. https://www.nature.com/scitable/topicpage/myelin-a-specialized-membrane-for-cell-communica Variation in the speed of nerve impulses Myelination is another factor which affects the speed at which nerve impulses are transmitted. The axon fibres of neurons can be mylinated or unmylinated. This refers to a lipid-rich substance surrounding the nerve fibre. This coating is provided by a series of Schwann cells with gaps between called nodes of Ranvier. In mylinated nerve fibers the Variation in the speed of nerve impulses The presence of a myelin sheath surrounding the axon fibre increases the speed of transmission of the action potential. Only at the junctions in the sheath (nodes of Ranvier) the axon membrane exposed. Elsewhere along the fibre, the electrical resistance of the myelin sheath prevents depolarization of the nodes. http://classroom.sdmesa.edu/eschmid/F09.07.L.150.jpg Non-myelinated myelinated The action potentials actually ‘jump’ from node to node (this is called saltatory conduction, meaning ‘to leap’)This greatly Variation in the speed of nerve impulses Use the website below to investigate the effect of axon diameter and myelination on velocity of signal transmission. Carry out the simulation with unmyelinated fibres. What is the effect of axon diameter on speed? Then repeat the experiment with myelinated fibers. What is the effect of myelination on speed? Suggest why, for very small axon diameters, there is little benefit of myelinated velocity. /ilearn.med.monash.edu.au/physiology/action-potentials/axon-diameter#simulat napses as junctions between neurons A synapse is the link point between neurons. A synapse consists of the swollen tip (synaptic knob) of the axon of one neuron (pre-synaptic neuron) and the dendrite or cell body of another neuron (post-synaptic neuron). At the synapse, the neurons are extremely close but they have no direct contact. Instead there is a tiny gap, called a synaptic cleft, about 20 Pre-synaptic neuron nm wide Post-synaptic neuron Synaptic cleft http://www.iflscience.com/sites/www.iflscience.com/files/blog/%5Bnid%5D/ napses as junctions between neurons ere are 3 main types of synapses: Synapses between neurons, in both the brain and spinal cord. Synapses between neurons and muscle fibres (effectors). Synapses between neurons and glands https://kypho.com/types-of-synapse napses as junctions between neurons Another (4th type) of synapse can be found between neurons and sensory receptors of sense organ (e.g. sense organ of the skin)… napses as junctions between neurons An action potential cannot cross the synaptic cleft between neurons – the nerve impulse is carried by chemical transmitter substances called neurotransmitters. These chemicals are made in the Golgi apparatus of the cell that is sending the impulse (the pre- synaptic neuron) Release of neurotransmitters from a presynaptic membrane 1. A nerve impulse (action potential) reaches the terminal end of the pre-synaptic neuron. 2. Depolarisation causes voltage gated calcium channels to open. Ca2+ rushes into the neuron. 3. Ca2+ causes synaptic vesicles containing a neurotransmitter to move to the membrane and fuse. Generation of an excitatory postsynaptic potential 4. Neurotransmitters, such as acetylcholine, that were stored in the synaptic vesicle now diffuse (from high to low concentration) rapidly across the synaptic gap.5. The neurotransmitter binds with the neuroreceptor (protein channels) in the post- synaptic membrane. 6. The proteins channels of the receptors open upon binding, allowing the influx of Na+ ions. An excitatory action potential is initiated, and the nerve impulse is propagated along the  https://www.quora.com/What-happens-first-when-a-neurotransmitter-binds-to-a-postsynaptic-neuro Generation of an excitatory postsynaptic potential Neuroreceptors are protein channels embedded in the cell membrane of the postsynaptic synapse. Upon binding of the neurotransmitter to the binding side of the receptor protein Na+ rushes into the neuron. Acetylcholine is one of the most common neurotransmitters and is used at many synapses, including etylcholine - Secretion and Reabsorption Acetylcholine is an organic molecule composed of acetyl CoA and choline. It acts as a neurotransmitter for both, PNS and CNS. In the PNS, it is the main neurotransmitter in the autonomic The transmitter substance on nervous system where it is the receptors is immediately responsible for activating muscles. inactivated by enzyme action to avoid overstimulation. The enzyme Acetylcholinesterase (AChE) hydrolyses ACh to choline and ethanoic/acetic acid, which are inactive as transmitters. As a consequence, the ion channel of the receptor protein closes, and the resting potential in the post-synaptic